Physicochemical and Electrochemical Properties of the Organic Solvent Electrolyte with Lithium Bis(fluorosulfonyl)Imide (LiFSI) As Lithium-Ion Conducting Salt for Lithium-Ion Batteries

Abstract

In this study, we investigated the physicochemical and electrochemical properties of LiFSI solution comparing with those of LiPF6 in EC/DEC (3/7, v/v), and discuss the difference in ionic conductivity between these electrolyte solutions based on self-diffusion coefficients measured by PFG-NMR. Self-diffusion coefficients of solvent molecules and ions in 1M LiFSI solution are about 1.5 times larger than those of 1M LiPF6 solution. On the other hand, the degree of dissociation of LiFSI estimated by the Nernst-Einstein Equation from measured ionic conductivity and self-diffusion coefficients of Li+ and FSI- is lower than that of LiPF6. Therefore it is strongly suggested that almost the same ionic conductivity of 1M LiFSI solution as that of 1M LiPF6 in EC/DEC (3/7, v/v) is the result of cancelation of these two factors in the opposite direction.

Self‐Diffusion Coefficients in Solutions of Lithium Bis(fluorosulfonyl)imide with Dimethyl Carbonate and Ethylene Carbonate

“…In the literature, there are only few reports on the diffusion coefficients of systems containing LiFSI. Takekawa et al report self‐diffusion coefficients for a 1 M solution of LiFSI in the solvent mixture ethylene carbonate‐diethyl carbonate (volume ratio 3/7). Tominga and Yamazaki report self‐diffusion coefficients for LiFSI solutions in poly(ethylene carbonate).…”

Diffusion of Lithium Cation in Low-Melting Lithium Molten Salts

“…(a) Applicability of the Stokes–Einstein equation in 1:1 lithium molten salts and (b) a close-up view of the region occupied by low-melting salts: red □, LiF; , red ◇, LiCl; , red △, LiNO 3 ; , red ●, Li­[FSA]; red ■, Li­[FTA]; red *, Li­[HFIP] ( n = 7.2); red +, Li­[PFP] ( n = 7.2); and mixtures: green ○, Li­[FSA]-G3; green □, Li­[FSA]-G4; purple ○, Li­[FSA]-[P13]­[FSA]; blue ○, Li­[FSA]-EC-DEC …”

“…The σ calc using the Nernst–Einstein equation is derived for noninteracting ions, as in an infinitely dilute solution . The σ calc and experimental ionic conductivity ,− , (σ exp ) are plotted in Figure . The high-temperature lithium salts have very high σ exp values; however, they are lower than the expected value from the D because of ionic association.…”

“…Walden plot of log­(molar conductivity, Λ) against log­(reciprocal viscosity, η –1 ) for 1:1 lithium molten salts: red □, LiF; red ◇, LiCl; red △, LiNO 3 ; red ●, Li­[FSA]; red ■, Li­[FTA]; red ×, Li­[TFSA]; red *, Li­[HFIP] ( n = 7.2); red +, Li­[PFP] ( n = 7.2); and mixtures: green ○, Li­[FSA]-G3; green □, Li­[FSA]-G4; purple ○, Li­[FSA]-[P13]­[FSA]; blue ○, Li­[FSA]-EC-DEC; blue □, Li­[FSA]-AN …”

“…To elucidate the unique rate performance versus the poor transport property, the self-diffusion coefficients ( D ) of the Li + ( D (Li + )) and counteranion ( D (Li + )) for the Li­[FTA] are required because it is a significant parameter for the mobility of ions and intimately interrelated with the viscosity and ionic conductivity. The D values for the various electrolytes of the lithium secondary battery, which are a mixture of the lithium salts and solvents, such as organic solvent or RTILs, were reported in many papers − by a pulsed-gradient spin–echo nuclear magnetic resonance (PGSE-NMR) method at room temperature. In contrast, the D values of the conventional lithium molten salts at high temperature (above 300 °C), such as halides or nitrates used in a thermal battery, were mostly measured by a capitally method or molecular dynamics simulation. , Recently, the PGSE-NMR method was performed at high temperature for a few alkali metal molten salts. − …”